Wipe down the inside with a damp cloth to remove any dust or dirt. If it's really grimy, use a degreaser. Cover the insides of the doors and cabinet with primer, using a disposable chip brush and short, erratic strokes. Keep the brush pretty dry; you want the undercoat to peek through to give the interior a clean yet worn appearance.
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Medium-grit sanding sponge
3 disposable chip brushes
1 quart of primer for the interior2 quarts of latex paint for the exterior